feat: support project-aware global shims for vp-installed global CLIs

## Summary
Add support in vite-plus for project-aware global shims for CLIs installed via `vp i -g`.

## pnpm reference
pnpm documents project-aware global bins here:
https://pnpm.io/global-packages#project-aware-global-bins

From the docs, the relevant behavior is:
- globally installed packages are isolated from each other
- bin shims are generated in the global bin directory
- those shims point into the appropriate install group
- the invoked CLI can operate with awareness of the current project context

## What we want for vite-plus
For CLIs installed via `vp i -g`:
- global shims should be available as normal executables
- when invoked inside a project, the shim/runtime should resolve against the current project context where appropriate
- the behavior should feel similar to local project bins, but for globally installed tooling

## Open questions
- Should vite-plus mirror pnpm's approach closely, or adapt it to vp's own installation/runtime model?
- What project context needs to be discovered at execution time?
- Are there edge cases around resolution, workspace roots, or cross-platform shim generation?

Start with the pnpm project-aware global bins reference and trace how `vp i -g` currently installs global CLIs. Define the project context, resolution behavior, workspace-root handling, and cross-platform shim expectations before identifying implementation points. Done means globally installed CLIs behave as normal executables while resolving project context appropriately.

Written by the indexing model from the issue text.